Accelerant
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Accelerants, or accelerators, are substances that increase the rate of a natural or artificial chemical process. They play a major role in chemistry, as most chemical reactions can be hastened with an accelerant. They are commonly used in contexts such as fire investigation where they can indicate arson, in construction to speed the curing of building materials, and in sulfur vulcanization to produce rubber products such as tyres. In fire investigation, accelerants are often detected through laboratory analysis of fire debris. Various types of accelerants exist, including liquids, solids, and gases, each with specific properties and applications. Understanding accelerants is crucial in forensic science, engineering, and other fields where controlled chemical reactions are essential.
Accelerants function by either altering a chemical bond, speeding up a chemical process, or changing the reaction conditions. Unlike catalysts, accelerants may be consumed during the process.
Vulcanization[edit]
The use of accelerators and activators lowers the activation energy of a vulcanization reaction from 210 kJ/mol to 80–125 kJ/mol, which is necessary if sulfur is used alone. Accelerators and activators break sulfur chains. Accelerated sulfur vulcanization requires only 5–15 sulfur atoms per cross-link, compared to 40–45 sulfur atoms per cross-link for non-accelerated sulfur vulcanization. There are many accelerators available for the vulcanization of rubber due to the wide range of rubber products on the market with a wide variety of properties. There may be up to eight different rubber compounds in a car tyre, each with specific properties. For instance, the tread in a typical passenger car tyre consists of a mixture of styrene-butadiene rubber (SBR) and butadiene rubber (BR). The tread should have high abrasion resistance and grip on both dry and wet roads. The side wall of the tyre should have high flexibility, meaning it should resist repeated flexing during operation without cracking. It normally consists of a mixture of natural rubber (NR) and butadiene rubber. Inside the tyre is a rubber compound whose function is adhesion between the rubber and the steel cord of the belt. It typically consists of NR for stiffness, with a very high sulfur level (up to 8%) to promote adhesion with the steel cord. The base of the tyre structure is normally a mixture of NR, SBR, and BR. It should have a very good adhesion to the polyester cord, which is used as reinforcement. The inner side liner normally consists of butyl rubber (IIR) that has been halogenated. For each of these compounds, different accelerators and mixtures of accelerators must be used to obtain the required properties.[1]
A vulcanization accelerator is typically used in combination with sulfur as the cross-linker, and with zinc oxide and stearic acid as activators. The various types of rubber used in tyre compounds all have different vulcanization characteristics, like speed and extent of cure (the number of cross-links). A typical passenger car tyre is vulcanized for 10 minutes at 170 °C (338 °F).
Classification[edit]
There are two major classes of vulcanization accelerators, primary accelerators and secondary or ultra accelerators.
Primary[edit]
The main primary accelerator group used in tyre manufacture is the sulfenamides.[2] These are produced by an oxidative coupling reaction of mercaptobenzothiazole (MBT) with a primary amine such as cyclohexylamine or tert-Butylamine. Secondary amines like dicyclohexylamine may also be used but result in much slower accelerators. Such a slow accelerator is required in the steel cord adhesion compound, because for optimal adhesion a slow cure is required. Another important group of primary accelerators is formed by the thiazoles. The two main substances are MBT and MBT disulfide, a product formed by the oxidative coupling of two MBT molecules. The thiazoles are used for the vulcanization of thick parts and as basic accelerators in EPDM rubbers (ethylene-propylene-diene), in combination with mixtures of ultra-accelerators.
In the vulcanization of neoprene (polychloroprene or CR) rubber, the choice of accelerator is governed by different rules than other diene rubbers. Most conventional accelerators are problematic when CR rubbers are cured[why?] and the most significant accelerator is ethylene thiourea (ETU) which has been classified as reprotoxic. The European rubber industry has begun a research project titled SafeRubber to develop a safer alternative to the use of ETU.[3]
Secondary[edit]
The main categories of secondary or ultra-accelerators are thiurams and dithiocarbamates. In tyre vulcanization, they are used as a small addition to sulfenamides to boost the speed and state of cure.[4] They have a very fast vulcanization speed and therefore, next to boosters in tyre compounds, are used as main accelerators in EPDM and latex compounds. EPDM rubber has much fewer cure sites than natural rubber or SBR and therefore needs a rapid vulcanization system to have sufficient cure speed. Latex is cured at relatively low temperatures (100–120 °C (212–248 °F)) and therefore requires a rapid accelerator.[why?] The major thiurams used are tetramethylthiuram disulfide (TMTD) and tetraethylthiuramdisulfid (TETD). They are produced by the reaction between dimethylamine or diethylamine and carbon disulfide. The major dithiocarbamates are zinc salts, zinc diethyldithiocarbamate (ZDEC), and zinc dibutyldithiocarbamate (ZDBC).
Cement and concrete[edit]
Cement accelerators are available as admixtures for use in concrete, mortar, rendering and screeds. The addition of an accelerator speeds the setting time and thus curing starts earlier.[5] This allows concrete to be placed in winter with reduced risk of frost damage.[6] Concrete is damaged if it does not reach a strength of 500 pounds per square inch (3.4 MPa) before freezing.[7] Typical cement accelerators are calcium nitrate (Ca(NO
3)
2), calcium formate (Ca(HCOO)
2) and sodium nitrate (NaNO
3).[8]
Fire[edit]
In fire protection, the term accelerant is used differently from its use in chemistry, to refer to any material that initiates and promotes the development of fire, including in cases of arson, whether a chemical or not. Chemists distinguish an accelerant from a fuel, such as gasoline.
A fire is a self-sustaining, exothermic oxidation reaction that emits heat and light. When accelerants such as oxygen-bearing liquids and gases (like NO
2) are used, fires produce more heat, consume fuel more quickly, and spread quicker. Fires involving liquid accelerants like gasoline burn quicker, but at the same temperature as fires involving ordinary fuels.
Fire investigation[edit]
Indicators of an incendiary fire or arson can lead fire investigators to look for the presence of fuel traces in fire debris. Burning compounds and liquids can leave behind evidence of their presence and use.[9] Fuels present in areas where they are not typically found can indicate an incendiary fire or arson. Investigators often use special detection dogs known as accelerant detection canines trained to detect liquid accelerants. Well-trained dogs can pinpoint areas for the investigator to collect samples. Fire debris submitted to forensic laboratories employ sensitive analytical instruments with GC-MS capabilities for forensic chemical analysis.
Types[edit]
Many accelerants are hydrocarbon-based fuels, sometimes referred to as petroleum distillates, such as gasoline, diesel fuel, kerosene, turpentine, butane, and various other flammable solvents. These accelerants are also known as ignitable liquids. Ignitable liquids can leave behind irregular patterns on surfaces, which can indicate the presence of an ignitable liquid in a fire and its point of origin. However, irregular patterns may also be found in fires involving no accelerant, particularly in cases of full-room involvement.
The properties of some ignitable liquids make them dangerous fuels. Many ignitable liquids have high vapour pressures, low flash points, and a relatively wide range between their upper and lower explosive limit. This allows ignitable liquids to ignite easily, and when mixed in a proper air–fuel ratio, readily explode.
Common household items and objects can be accelerants. Wicker and foam have high surface-area-to-volume ratios and favourable chemical compositions and thus burn easily and readily. Large fuel loads can increase the rate of fire growth as well as spread the fire over a larger area, thus increasing the amount of fire damage. Unusual amounts and types of fuel in a particular area can indicate arson. Whether available combustible materials constitute accelerants depends on whether they were intentionally burned.
See also[edit]
References[edit]
- ^ Roberts, A. D. (1988). Natural Rubber Science and Technology. Oxford University Press.
- ^ Koval', I. V. (1996). "Synthesis and application of sulfenamides". Russian Chemical Reviews. 65 (5): 421–440. Bibcode:1996RuCRv..65..421K. doi:10.1070/RC1996v065n05ABEH000218. S2CID 250881411.
- ^ "SafeRubber, an alternative for accelerators in the production of rubber". SafeRubber. Archived from the original on 2016-07-09.
- ^ Engels, Hans-Wilhelm; et al. "Rubber, 4. Chemicals and Additives". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a23_365.pub2. ISBN 978-3527306732.
- ^ Justnes, H. (2000): Accelerator Blends for Portland Cement. Proceedings of Cement and Concrete Technology in the 2000s, September 6–10, 2000, Istanbul, Turkey, Vol. 1, pp. 433–442
- ^ ACI 306R-88: Cold Weather Concreting. "Cold Weather Concreting" (PDF). Concrete Contractors Association of Greater Chicago. Archived from the original (PDF) on 2011-07-25. Retrieved 2011-03-05.
- ^ Korhonen, Cortez & Durning 1997, p. 19.
- ^ Korhonen, Charles J.; Cortez, Edel R.; Durning, Timothy A. (1997). "Antifreeze Admixtures for Concrete". Cold Regions Research and Engineering Laboratory. Special Report 97-26. ISBN 9781428913158.
- ^ International Association of Arson Investigators, Massachusetts Chapter (1999). A Pocket Guide to Accelerant Evidence Collection (2nd ed.). The Association.
